First time mount perfect!!!

I’ve gotten that lucky. 

I can set up Decca cartridges by eye but damned if I can get an ancient NAD 9200 right.

Yes dgarretson with headshells you can make a mounting jig that will locate the cartridge perfectly every time. The only outlier is VTA which you still have to dial in. I have them for my Kuzma arms. I had to make two of them because the offset angle is not the same on all their 4 point arms. You can do it for fixed head shells but it is not as easy. I must say that given the time it takes to make a jig I can align 3-4 cartridges the old fashion way so unless you are a reviewer a flipping cartridges all the time it may not be a time saver.

Since my Kuzma 4PT and SME 3012R both have removable headshells, I can usually replicate perfect cartridge/headshell geometry on the workbench under a large magnifier.  Slide 'em into the tonearm and good to go.   

Hi,
never, but close 2 cartridges before last one. Check again you may missed something, cantilever, sra, weight.....